At present, non-steroidal anti-inflammatory drugs (NSAIDs), non-narcotic analgesics, narcotic analgesics, and the like are used against general “pains,” so that therapeutic methods therefor have begun to be established. However, there are hardly any analgesics that can satisfy the neuropathic pains under the current situation.
“Pains” are roughly classified by causes of diseases into nociceptive pain (so-called general “pain”) caused by a strong stimulus (nociceptive stimulus) that would result in damages to body tissues, and neuropathic pain (neurogenic pain), which is a disease pain resulting from an injury or malfunction of the central or peripheral nerve. This neuropathic pain causes, in addition to a spontaneous pain, a symptom such as a hyperalgesia that lowers the pain thresholds against the nociceptive stimulus and a severe pain (allodynia) caused by tactile stimulation that usually does not induce the pain. Once the morbid state is completed, it turns chronically whereby the outcome is very intractable.
As a result of intensive studies on the compounds that show effects on various pains, the present inventors have found that the amino acid derivative of the present invention has excellent analgesic actions to not only a nociceptive pain model animal but also a neuropathic pain model animal. As amino acid derivatives, N-cinnamoyl-tryptophan as an intermediate of a compound having an anti-allergic action (Non-Patent Publication 1), N-cinnamoyl-L-tryptophan, N-cinnamoyl-D-tryptophan, and N-3-chlorocinnamoyl-tryptophan that suppress excitation of gigantic neural cells of East African land snail (Non-Patent Publication 2), p-coumaroyl-L-tryptophan and caffeoyl-tryptophan, which are isolated substances from coffee beans (Non-Patent Publication 3), N-acrylyl-L-tryptophan and N-acrylyl-L-leucine, which are monomers of a copolymer (Non-Patent Publication 4) are disclosed; however, any one of the publications do not describe at all that these compounds are useful as pharmaceutical agent, and especially as analgesics. In addition, Patent Publication 1 describes p-coumaroyl-L-tryptophan, N-caffeoyl-L-tryptophan, p-coumaroyl-L-tyrosine, or the like that is extracted from a plant; however, the publication does not describe that these have analgesic actions, and the like.